High-accuracy current measurement in automotive applications

New technology provides measurement and control options

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Figure 1: Transducer accuracy

The automotive industry has been undergoing a major transformation during the past few years. After more than a century of improving and refining the traditional gasoline and diesel engines that power automobiles, car manufacturers are now increasingly supplementing this traditional technology with some form of electrical boost. These so-called "hybrid" vehicles are capable of meeting or exceeding customer expectations in terms of power and response whilst consuming less fuel and hence emitting fewer exhaust gases. The new electrical systems in these vehicles, including braking energy recovery systems, start-stop capabilities and electric motors driving the wheels, all require accurate measurement and control of the electricity flowing in the vehicle to optimize their performance and avoid a catastrophic failure. An essential part of these systems is the battery current sensor that measures the battery's charge and discharge level, and its state of health. There are several existing technologies for making a good automotive battery current sensor. The shunt has been the choice of some car manufacturers, while others prefer to use Hall-effect or fluxgate sensors in their designs. As with most things, each technology has its advantages and its drawbacks. Shunt-based sensing Shunt-based current sensors have been used widely in the last decade to measure battery currents (predominantly in premium-segment cars). These cars employ advanced electronics to accurately track the battery's charging capacity and overall health, and sometimes some form of performance-enhancing electrical assistance. The shunt is a resistor made of relatively expensive materials such as Manganese or Nickel-Chrome alloys, whose impedance is very low, well known, and precisely characterized over a range of temperatures and voltages. By measuring the voltage drop across the shunt resistor, the current flow through the resistor can be calculated using Ohm's Law. This resistor is placed in the path of current flow to and from the car's battery, providing accurate, high-resolution information about the voltage and current (a temperature sensing feature is also added). Additionally, they can measure a very wide range of current amplitudes, from milliamperes to over one thousand amperes in short bursts (experienced when the car starts). Shunts do have a problem when measuring very high currents, as they must be proportioned to accept the high current flows, and they also dissipate significant power. These advantages and the absence of equivalent-performance alternatives over the past decade have made them a preferred choice for premium-market car makers, albeit at a relatively high unit cost. Hall-effect sensing Hall-effect-based current sensors have been around in industrial applications for several decades, and have also in the automotive industry for many years as well. Hall-effect sensors are sensitive to magnetic fields. By concentrating the magnetic fields generated by the currents passing in the battery cable on the sensor's Hall-cell, for example, the sensor will output a signal that is proportional to the passing current. This signal can then be further processed in the analog or digital domain to eliminate noise, and to compensate for the errors inherent to the technology. An analog voltage output, or some form of PWM, or SENT signal output can then be provided to the vehicle's battery management processors, which, in turn will integrate it to determine the charge and/or battery health. One of the characteristics of Hall-effect sensors that have prevented some carmakers, especially premium-market ones, from using them in their battery-management systems is their offset error. The electrical- and magnetic-offset errors increase the uncertainty of the measured signal. They cannot be fully compensated and can thus affect the calculations of battery-charge levels. Offset error is also temperature dependent, sometimes with significant part-to-part variations. Shunt-based sensors lack these magnetic hysteresis effects. Overall errors of 3% to 5% were common in Hall-cell sensors of only a few years ago. The latest technological improvements reduce this error from 1% to 2% in many cases. Chip- and magnetic-core designs incorporating Hall-effect sensors are the main drivers of this improvement. (As a comparison, shunts have an accuracy of about %.) Hall-effect sensors have several advantages over shunt-based sensors, the biggest ones being isolation and reliability. Hall-effect sensors are galvanically isolated from the primary current since they are placed near their target, and capture magnetic field information through space to create a reading. In this application Hall-effect sensor ICs are placed around the cable. They can also withstand much higher current and voltage peaks than other subsystems without sustaining damage. Since placement of Hall-effect sensors is not limited to the battery terminal post (as it is for most shunts); they can be placed anywhere along the cable or conductor whose current needs to be measured. This provides car manufacturers with significant economic benefits because they can source a single, standard component for use in any and all different engine and battery configurations with different cable lengths. Hall-effect sensors are also generally much less costly to manufacture than equivalent shunts. Shunts employ high quantities of expensive materials and electronics to filter and condition the output signal, whereas Hall-effect sensors only require modest amounts of ferrous material and an integrated circuit (ASIC). If a shunt needs to provide a galvanically-isolated output, considerable additional cost needs to be added to the product. The compelling cost-benefit of Hall-effect technology has made many car manufacturers choose this technology for their battery current sensing applications, in exchange for modest reduction in accuracy, as compared to shunt-based alternatives. Fluxgate technology Fluxgate technology bridges the gap between Hall-effect sensors and shunts by providing the advantages of an isolated sensor with negligible signal-offset. For many years this has been a technology used in expensive industrial components, but fluxgate current sensors are now available in automotive-qualified solutions at comparable costs to the shunt. Much like Hall-effect sensors, they are sensitive to the magnetic fields generated around the primary cable, but their measurement principle is unique, and any signal offset is automatically cancelled by the alternating currents in the windings of the solid magnetic core. A fluxgate sensor's measurement error is less than 0.5% with a global offset lower than 10 mA in a 400 A-range product, giving them an inherent advantage (see Figure 1). When integrating current values over time to obtain the state-of-charge calculation in a vehicle, for example, the impact of the improved accuracy is multiplied, giving fluxgate sensors a clear advantage over Hall-effect, and even shunt-based technologies (see Figures 2 and 3). They can be also placed anywhere along the conductor, close to or far away from the battery, and thus offer flexibility of design to car manufacturers that helps them reduce costs. Fluxgate sensors are ideal for hybrid- and electric-vehicles where accurate current measurement is critical, and where the high-current amplitudes and sensor isolation pose a challenge to shunt-based alternatives.

Several technologies exist for measuring automotive battery currents, each with particular advantages and disadvantages. Premium-segment car makers have hitherto preferred shunt-based solutions due to their high accuracy and wide measurement range. Fluxgate technology, once reserved for very high-end industrial applications, has now been re-engineered to become price-competitive for automotive applications, and offers better accuracy than shunts whilst remaining galvanically isolated. When cost remains the biggest priority, Hall-effect sensors are the ideal solution. Their main historical drawback (offset errors) is being addressed with technological advances to provide accuracy in a/ low-cost product. The increasing need for electrical current measurements in cars gives an opportunity for these various technologies to further develop and become an essential element in the design and production of future automotive systems. LEM